The present invention relates to an improved process for improving the yields of fibrous materials, such as wood pulp, and of increasing the viscosity of the delignified pulp, during an oxygen-alkali delignification or bleaching treatment. In the process, conducted at relatively low consistency, there are present small amounts of ammonia or a compound which will release ammonia under the alkaline conditions of the oxygen-alkali delignification.
Current interest in the paper industry has emphasized the desirability of delignifying wood pulp and other cellulosic fibrous materials using oxygen-alkali treatments. Such processes are desirable because they avoid the use of the traditional chlorination bleaching which uses a more expensive chemical and introduces complications due to the need to remove chlorine-containing by-products from the effluent streams. This requires expensive chemical recovery systems so as to abate stream and environmental pollution problems. A number of oxygen-alkali delignification processes have been presented, such as those of Richter U.S. Pat. No 1,860,432; Graangard et al. U.S. Pat. Nos. 2,926,114 and 3,024,158; Gaschke et al. U.S. Pat. No. 3,274,049; Meylan et al. U.S. Pat. No. 3,384,533; Watanabe U.S. Pat. No. 3,251,730; Rerolle et al. U.S. Pat. No. 3,423,282; Farley U.S. Pat. No. 3,661,699; and French patents 1,130,248 and 1,387,853. A more recent process which has been found to be particularly advantageous is that of Roymoulik et al. U.S. Pat. No. 3,832,276.
The Roymoulik et al. patent process is a highly satisfactory process. However, the paper industry continues to strive to improve pulp yields and to improve the quality of the pulp. It is known, for example, that oxygen-alkali delignification or bleaching methods tend to reduce the pulp viscosity. To counteract this problem, and to strive for optimum yields, various additives have been suggested in the oxygen-alkali treatments, such as magnesium salts, chelating agents and sequestering agents, have been employed in the systems in order to reduce the cellulose degradation which causes the unwanted reduction in pulp viscosities.
Yield loss during delignification or bleaching is not unique to oxygen-alkali delignification processes. It is encountered in many pulp treatments of cellulosic materials. With the increasing costs of the source of cellulosic materials, such as tree logs, the reduction of yield losses becomes of increasing importance to the paper industry. The search for means of reducing yield losses and viscosity losses has been going on for many years, even going back prior to the advent of oxygen-alkali delignification treatments.
One such suggested means of preventing viscosity reduction due to cellulose depolymerization or degradation is Heald U.S. Pat. No. 3,368,935. That patent is directed to a sulfite pulping process where urea is employed in the cooking liquor to control pH and prevent depolymerization of the cellulose. Sulfite cooking liquors are, however, an entirely different system from an oxygen-alkali bleaching system. The reaction mechanisms in the two processes are entirely different.
Baudisch U.S. Pat. NO. 2,271,218 describes a method of producing textile fibers by treating corn straw in a solution of alkali and urea. The process is a two-stage pulping process in which corn straw is treated with alkali and urea in the first stage and defiberized mechanically in the second stage. Such a treatment is quite different from an oxygen-alkali delignification treatment.
Liebergott et al. U.S. Pat. No. 3,740,311 describes a process for the delignification of wood pulp in which the pulp is treated at a relatively high consistency with ammonia and oxygen. In the process, ammonia is the sole source of alkalinity. Gaschke et al. U.S. Pat. No. 3,274,049 is similar to the Liebergott patent and differs in that it is directed to the treatment of bagasse, but it also employs ammonia as the sole source of alkalinity.
A publication by Lyman C. Aldrich, "Cellulose Degradation Inhibitors for the Chlorination Stage," TAPPI, Vol. 51, No. 3, pp. 71A-74A (March 1968) describes the use of urea and ammonium hydroxide (actually ammonium chloride, which is formed by the instantaneous reaction with ammonium hydroxide on the chlorine-containing acidic treatment liquor) during chlorine bleaching to inhibit cellulose degradation. While the addition of urea and ammonium hydroxide improves pulp viscosity to some extent, it also produces pulps having higher permanganate numbers, reflecting a higher lignin content than the controls. Thus, in the chlorination bleaching treatment of the Aldrich publication, the addition of urea and ammonium hydroxide at levels of additive above 0.25% actually reduced the amount of delignification. This is a serious disadvantage in a process whose sole purpose is to remove lignin from the pulp and so that if any subsequent chemical treatment is involved, such as the use of chlorine dioxide in a subsequent treatment of the pulp, that treatment can be minimized. In the acidic chlorination conditions employed by Aldrich, the nitrogenous compounds react with the chlorine in the bleaching solutions to form chloro derivatives of the nitrogen compound, thus wasting bleaching chemical. Thus, the addition of urea or ammonia serves no useful purpose insofar as delignification is involved.
A paper by Tobar, "Sulfamic Acid in the Chlorination and Hypochlorite Bleaching of Pulp," TAPPI, Vol. 47, No. 11, pp. 688-691, and Tobar U.S. Pat. No. 3,308,012, describe the use of urea and ammonium chloride as cellulose stabilizers during alkaline hypochlorite bleaching. On page 691 of the publication, Tobar states that in a hypochlorite bleaching system, the addition of urea or ammonium chloride produced both a loss in brightness and a loss in pulp viscosity, thereby showing that these agents in such a system actually produce undesirable results.
Gudivaka et al., "Inhibitors in Pulp Bleaching", Indian Pulp and Paper, pp. 447-452, (January 1971) describe the use of urea, ammonia and sulfamic acid in the bleaching of pulp by chlorine or hypochlorite. Such a system is, of course, quite different chemically from oxygen-alkali bleaching system. The results obtained by the authors were inconclusive.
Viscosity represents a measurement of the average degree of polymerization of the cellulose in the pulp sample, i.e., the average chain length of the cellulose. Thus, decreases in viscosity values represent the extent of depolymerization or degradation caused by the bleaching process. Excessive degradation is to be avoided since it provides undesirable physical properties in any paper made from the pulp.
Kappa No. is determined by the potassium permanganate consumed by a sample of pulp and represents a measurement of its retained lignin content. The higher the Kappa No., the less bleached and delignified is the pulp. By comparing Kappa Nos. of samples before and after bleaching treatment, one can obtain an evaluation of the extent of delignification which has taken place.
It is, therefore, an object of the present invention to provide an improved process for the delignification or bleaching of pulp by the oxygen-alkali process.
It is another object of the present invention to provide a method of delignifying wood pulp which will reduce yield losses and provide a pulp of inmproved viscosity.
Further objects will be apparent to those skilled in the art from the present description, taken in conjunction with the appended drawing, which is a graph comparing the shrinkage or yield loss in the oxygen-alkali treatment of wood pulp with various concentrations of ammonia in the pulp.